COLLOIDS A AND

Colloids and Surfaces SURFACES ELN EV I ER A: Physicochemical and Engineering Aspects 123-124 (1997) 651-655

Neglected aspects of food flavor perception

M. Larsson, K. Larsson

Department of Food Technology, Chemical Center, University of Lund, P.O. Box 124, S-221 O0 Lurid, Sweden

Received 30 July 1996; accepted 6 August 1996

Abstract

The olfactory receptors play a major role in perception of food flavor, owing to their sensitivity and wide diversity. It is generally accepted that the transport of flavor molecules from the oral cavity to the olfactory region takes place in the gas phase. It is proposed that an additional transport can take place via a surface film. Non-volatile flavor molecules can spread over the mucus layer in the nasal cavity fast enough, provided that they are surface-active. During this transport they may be associated with the proline-rich saliva proteins, which exhibit a high pressure of monolayer spreading. In that way certain flavor molecules are proposed to reach the olfactory receptors even if they have extremely low vapor pressure. The chemoreceptor system in the nasal cavity also consists of the vomeronasal receptor region. Unlike the olfactory receptors their signals can only enter the limbic part of the brain. Consequences of the unconscious component of flavor are considered, particularly as a possibility of providing an innate defence against toxic food components or infected food.

Keywords: Food flavor perception; Olfactory receptors; Chemo receptor system; Vomeronasal receptor

1. Introduction

Our sensory perception of food/food flavor involves the chemoreceptor system, taste and smell, and also to some extent, the nerve endings, record- ing temperature and mechanical pressure. It is generally accepted that flavor substances reach the chemoreceptors via two routes; the taste buds on the tongue are reached via the liquid state (solution in the saliva), whereas the olfactory receptors are reached by compounds transported in the gas phase, [1] . We will present evidence indicating that non-volatile flavor components may also be carried via an aqueous surface film up to the olfactory epithelial cells. To the best of our knowl- edge, such a transport mechanism involved in the perception of food flavor, has not been consid- ered before.

Two additional aspects of flavor will also be

discussed. One is the possibility of an unconscious component of our attitude to food, based on activation of the vomeronasal system and the other concerns the possibility that instinctive reactions induced by food flavor act as an innate defence line against toxic agents and micro-organisms.

2. Full taste is achieved when we swallow

A main argument behind the assignment of a vapor phase transport of a roma components to the olfactory receptors is the reduction or elimina- tion of flavor perception when we taste a food with our nostrils closed. As discussed in the next para- graph there are reasons to also consider transport of non-volatile compounds to the olfactory region. On this basis it is proposed that flavor perception is not only a sum of signals from molecules trans-

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652 M. Larsson et aL / Colloids' Supfaces A." Physicochem. Eng. Aspects 123 124 (1997) 651-655

ported in the water phase to the taste buds and vapor-phase transport of molecules to the olfactory receptors, but also of signals from surface-film transport to the olfactory region. Such a mecha- nism will be presented below. However, first we will give some intuitive arguments, motivating the consideration of an additional transport route.

A piece of meat can be boiled for hours to remove all volatile compounds. Still it is possible to identify the meat flavor by swallowing the meat- saliva mixture. The meat flavor related to non- volatile compounds is considered to involve nucle- otides and peptides in the size range 1800 Da [1] . They can be divided into two groups. One contains more hydrophilic residues, and these peptides have been associated with desirable flavor, whereas the other group, consisting of peptides with hydro- phobic side-chains, is associated with off flavor. It is hard to accept that these drastic differences can be explained by different response combinations involving taste buds alone. The peptides with hydrophobic side-chains, however, are strongly surface active. This means that they can spread as a monolayer over the surface of the nasal cavity; either by themselves or associated with the most amphiphilic saliva protein fraction.

A cheese, well ripened, or the Swedish speciality "sour-herring" can taste delicious to some of us although we consider the strong smell very unpleasant. The bitter, salty, sour or sweet taste combined with the disagreeable odor of these foods can hardly explain the flavor; there must be some- thing more to it.

In the preparation of chocolate, the chocolate paste is melted and "conched" at about 60°C; a stirring process in air for hours to get rid of volatile off-flavors. Obviously all compounds with high vapor pressure will be strongly reduced by this process. The chocolate is then formed by crystalli- zation of the cocoa butter, forming a continuous matrix around dispersed sugar and cocoa powder particles. By reducing the amount of cocoa powder and increasing the conching time, a chocolate, quite pure with regard to volatiles, can be obtained. If we chew and swallow such a chocolate with the nostrils closed, it is still possible to sense a charac- teristic chocolate flavor. Again the four (or five if

counting amino acids) different kinds of taste sig- nals are not enough to explain this.

The final example, pointing in the same direc- tion, is wine. If we fill the mouth with wine and also inhale its vapor strongly, the total sensation is far from what is felt when the wine is swallowed.

Our chemoreceptor system, with the olfactory receptors as the most important component, is considered to be our phylogenetically oldest sense system. It does not seem likely that during evolu- tion, the ability of water-phase transport to these receptors has been completely lost and replaced by gas-phase transport. We suggest that these systems work in parallel.

3. On the possibility of monolayer transport of non-volatile flavor molecules in the saliva to the olfactory receptors - - a complement to gas phase transport

It is well known that the volatile compounds that contribute to flavor are lipophilic or amphi- philic in character, e.g. hydrocarbons, aldehydes, ketones, alcohols, carboxylic acids, esters etc. l-l]. A substance contributing to the flavor of white wines, for example, is isoamyl alcohol. It can form a monolayer on a water surface. When talcum powder is evenly distributed on a water surface in a Petri dish, and a droplet of isoamyl alcohol from a syringe is put in contact with the water surface, the particles are forced towards the wall within fractions of a second. Then after a few seconds the particles slowly move back when the alcohol is solved in the water phase. The process can be repeated until the solubility limit ofisoamyl alcohol in the water phase is reached. Lipids behave in a similar way. A droplet of a fatty acid, for example oleic acid, spreads fast as a condensed monolayer, and then the fihn is successively lost into the subphase. The film loss continues until the oleic acid solubility in water is reached. The velocity of such a film transport in the nasal cavity depends upon the spreading pressure (equal to the reduction in surface tension of the air/mucus interface).

A reasonable estimate of the time taken for a flavor substance to reach the olfactory region after swallowing by such monolayer transport is in the

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range 0.1-1 s according to simple model experi- ments. The flavor molecule might also form a complex with a very amphiphilic saliva protein component, which would act as a carrier in the monolayer state up to the olfactory receptors. It is easy to demonstrate the fast spreading of a mono- layer from saliva components on a water surface, by the movement of talcum powder distributed on a water surface when a droplet of saliva is applied.

An alternative mechanism of fast transport along the pharyngeal back-wall and the nasal cavity might be that saliva and flavor components together form a separate liquid phase. Their com- positions are so different that such two-phase separation seems reasonable. In that case the act of swallowing can induce a flush of the saliva layer above the mucus. The driving force for such a spreading process is the reduction in interracial energy, and the kinetic involved is also related to the viscosity of the spreading phase. For reasons given below, however, this alternative seems unlikely.

4. Amphiphilic saliva components expected to interact with flavor molecules

The surface tension reduction at the air/water interface by whole-saliva and various protein frac- tions from saliva has been examined by Vassilakos et al. [2] . When saliva is added to pure water or a physiological sodium chloride solution, the sur- face tension is reduced by as much as about 30 mN m -1. This explains the fast spreading of a saliva film over an aqueous surface, as mentioned above. The formation of an absorbed interfacial film, on the other side, when a new air/water interface is created from saliva solution, takes rather a long time. From a saliva solution of 5-20% (w/w) in water, the reduction in surface tension to reach a plateau value of about 30 mN m, takes about 20 min. This kinetic behavior is similar to that of most proteins and related to the rate of diffusion of the molecules to the interface and the rate of conformation rearrangement of the peptide chain at the interface. This process is too slow to be relevant to flavor perception.

Fractionation of saliva according to molecular weight has shown that two fractions [2] , with estimated molecular weight ranges 205-39 and 14-4.5 kDa respectively, gave the strongest reduc- tion in surface tension. Both were proline-rich (22.4 and 25.6% of the amino acids). More than 20 different proline-rich proteins have been demon- strated in human saliva, and this family of proteins is unique to saliva although small amounts have been observed in respiratory and pancreas fluids [3] . The functions of these proteins have not been established. Some are anionic and supposed to inhibit apatite crystal growth (saliva is supersatu- rated with regard to calcium and phosphate ions). Their strong surface-activity indicates that they might be involved as monolayer carriers of flavors. In general the spreading pressure of the saliva proteins is about 50% higher than that of proteins. In considering the possibility of monolayer trans- port of flavor substances to the olfactory region, it is necessary that other amphiphilic compounds in the saliva-food mixture do not interfere. We are not aware of any component that can compete with these saliva proteins. There are also lipids in the saliva [4] , but the concentration (about 0.1% (w/w)) is probably too low for monolayer forma- tion in competition with the saliva proteins.

If a monolayer transport process over the nasal mucus surface takes place in connection with swal- lowing, this surface film must disappear fast enough to provide the spreading pressure necessary for the next cycle of swallowing - monolayer transport - clearance.

The mechanism behind the surface clearance is proposed to be based on the same kind of film loss owing to solubility as described above, exem- plified by fatty acid monolayers. The ciliary move- ment of the mucus surface zone must be expected to make this monolayer loss into the subphase faster and more efficient. Furthermore the renewal by secretion and flow of the mucus layer must be so fast that the solubility level for film loss is not reached, otherwise the perception of flavor mole- cules by this route of transport would fade out. It seems likely that the saliva molecules that form the transport monolayer are also carriers of the actual flavor molecules to the olfactory receptors, delivering them when the protein-flavor molecular

654 M. Larsson et al. /Colloids' Surfaces A. Physicochem. Eng. Aspects 123-124 (1997) 651-655

complex is dissociated. The alternative to mono- layer transport, mentioned above, involving a sepa- rate liquid film which does not mix with the nasal mucus phase, is less likely with regard to the requirement of film removal. Clearance of the surface from such a macroscopic layer can hardly take place quickly and efficiently enough.

5. A purely unconscious component of flavor

The olfactory epithelial cells cover about 4 cm 2 of the upper part of the nasal cavity. They contain about ten million receptors of at least 1000 different kinds. The receptors activate G-proteins and the signals are transmitted to the cortex and the limbic system [5]. There is another group of receptor- containing epithelial cells at the bottom of the nasal cavity, the vomeronasal organ [6] . These receptors are activated by pheromones, which are known to influence social and sexual behaviour in animals. The vomeronasal receptors (also coupled to G-receptors) transmit their signals to the limbic system. They are extremely sensitive. From studies in insects it is known that one pheromone molecule is enough to produce a stimulus, whereas the olfactory receptors in humans require at least 50 odorant molecules [7] . As the nerve signals from this organ do not reach the cortex, unlike signals from the olfactory organ [6] , the effects are uncon- scious [2]. It has recently been realized that the vomeronasal organ is also well developed in humans, even if no human pheromone has been identified yet [2] . Pheromones, consist of fatty acids, steroids and other small amphiphilic mole- cules, e.g. peptides. Similar components occur in foods and it seems reasonable to ask whether some food compounds may activate this organ. If so, the effect on flavor perception would be of an emo- tional kind and provide an unconscious dimension to the sensory response.

6. On transport to the olfactory region via the circulation

Small molecules can easily penetrate the epithe- lial cell coat, particularly the sublingual region of

the oral cavity: nitroglycerin and nicotine are well- known examples. When extracted oat oil is applied on the skin, it is possible shortly afterwards to recognize in the mouth the characteristic oat lipid flavor. This was found by B. Dull [8]. The fast transport of dimethyl sulfoxide through the skin and the perception of in the mouth is well-known to most chemists. The perception of flavor in these cases involves the olfactory receptors. The release of very small aroma molecules which can penetrate the oral epithelium and enter the circulation pro- vides an additional route to reach the olfactory receptors. It is not likely, however, that it plays any major role in food flavor perception.

7. Flavor provides an innate defence system against toxic molecules and infected foods

Evolutionary changes of the perception of flavor have gone hand in hand with changes in eating habits as an important factor in survival; to detect food which is beneficial and avoid anything which is hazardous. Gastrointestinal infections have always been a scourge in the history of mankind, and fermentation of foods practised in all known cultures has been one way to reduce pathogenic micro-organisms. It has recently been found that beside the immune system there is a so-called innate defence line consisting of recently discovered anti-microbial peptides [9]. Bacteria grow too fast for the immune system to be able to fight them in time. A typical time taken to double the number of bacteria is 20 min, whereas cells in the immune system require about 20 h. These antimicrobial peptides in the epithelial mucus layers, exposed outwards, attack the bacteria directly, hence the name innate immunity.

We also have a "mechanical" innate defence line in the process of vomiting. This defence mechanism is activated when we ingest toxic food. The vomit- ing-centre in the medulla oblongata detects bacte- rial toxins and induces vomiting. Why expose the body to these toxins a second time? We propose here that there is an additional "psycho- mechanical" innate defence line based on reactions owing to experiences relating some gastrointestinal disorders and flavors of foods recently ingested.

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Thus, if during vomiting we recognize some special flavor, smoked salmon for example, it is likely that the next time we get exposed to smoked salmon, the vomiting reflex will be activated. These kinds of protective reactions are unconscious and explain why some people, without knowing the reason, just cannot eat certain foods. In a broader sense, such instinctive and direct reactions against toxic components or infected foods must have been favoured during evolution.

Acknowledgment

References

[1] G.I. Imafidon and A.M, Spanier, Trends Food Sci. Technol., 5 (1994) 315.

[2] N. Vassilakos, J. Rundegren, T. Arnebrant and P.-O. Glantz, Arch. Oral Biol., 37 (1992) 549.

I-3] A. Bennick, J. Dental Res. 66 (1987) 457. [4] B.L. Slomiany, V.L.M. Murty, A. Slomiany, Progr. Lipid

Res. 24 (1985) 311. [5] E. Buck, R. Axel, Cell, 65 (1991) 175. I-6] M. Halpern, Annu. Rev. Neurosci., 10 (1987) 325. [7] A. Menini, C. Picco and S. Firestein, Nature, 373

(1985) 435. 1-8] B. Dull, ConAgra Co., personal communication, 1996. [9] H.G. Boman, Annu. Rev. lmmunol., 13 (1995) 61.

This work is dedicated to Professor Stig Friberg, who has been a pioneer in introducing surface and colloid chemistry into the field of food flavors.